Clumping pop-corn convection

Hauke Schulz and Prof. Bjorn Stevens, researchers in the department “The Atmosphere in the Earth System” at the Max Planck Institute for Meteorology (MPI-M), show in their new study in the Journal of Atmospheric Science the first observational evidence of mechanisms, previously hypothesized based on modelling studies, leading to the self-aggregation of convection.

Clouds fascinate people in many ways and with the right amount of imagination clouds can look like animals, cars or the face of a good friend. Looking down on our planet from space, clouds also appear structured. This often has a rather simple explanation, as in the mid-latitudes, where we live, the atmosphere is disturbed by large-scale phenomena like frontal systems forcing clouds into specific pattern.

In contrast, the deep tropics are less disturbed by such synoptic weather variability, and can be idealized as a region, where the radiative cooling is balanced by randomly distributed moist convection, called radiative-convective-equilibrium. It is therefore surprising, that model simulations show that clouds often don’t persist in this randomly distributed, pop-corn-like state, but rather organize themselves to a confined, aggregated cluster without the disturbance of external organizing features, e.g. frontal systems. This aggregation process is referred to as convective self-aggregation.

Explaining this behavior of self-aggregation is interesting in its own right, but it has also implications on the mean state of the atmosphere. In a changing climate, the frequency and intensity of the self-organization might alter and thus impact the Earths’ climate. The processes of self-aggregation are also often associated with the formation of hurricanes when they form outside of the deep tropics.

Schulz and Steven’s test of model results is possible thanks to the long-term measurements at the Barbados Cloud Observatory, which samples air masses characteristic of the broader undisturbed tropics. The vast amount of data from bleeding edge instruments like the in-house developed Raman lidar enabled the researchers to perform this first observational analysis of processes believed to lead to convective self-aggregation, and provide an important touchstone for future modelling and theoretical studies.